Lighting devices of this type are generally devices which fulfil the dipped headlight or foglight function. Such devices comprise, in a known fashion, a reflector of the elliptical type, with a light source placed in a region of the first internal focus of said reflector, and a convergent lens whose focus is positioned in a region of the second external focus of said reflector.
Generally, in order to fulfil the main-beam function, a lighting device with a parabolic reflector is used which, using a light source placed at its focus, forms a light beam whose rays are for the most part substantially parallel to the axis of the device.
Currently, automobile optical units comprise a lighting device of the elliptical type in order to fulfil the dipped headlight or foglight function and a lighting device of the parabolic type for fulfilling the main-beam function, positioned side by side.
However, because the lighting device of the parabolic type fulfilling the main-beam function does not have a front lens, and because the lighting device of the elliptical type fulfilling the dipped headlight or foglight function has one, such an optical unit has a non-homogeneous external appearance.
In order to attempt to harmonise the external style of these optical units having a first lighting device fulfilling the dipped headlight or foglight function and a second lighting device fulfilling the main-beam function, it would be envisaged using, as a lighting device fulfilling the main-beam function, a lighting device of the elliptical type.
However, the lighting device of the elliptical type would not make it possible to form an efficient main-beam lighting beam.
This is because the value of the intensity formed by this type of lighting device in the axis thereof (the fundamental value for a lighting device fulfilling the main-beam function, insofar as it determines the "peak" of the main-beam light beam formed), is proportional to the apparent surface area of the optical system which, in the case of a lighting device of the elliptical type, is relatively small.
In order to make this apparent surface area bigger, it would then be necessary to increase the diameter of the convergent lens. However, in order to maintain a relatively limited lens thickness (and therefore weight), it is necessary to increase the focal distance of said lens.
In this case, the lighting device of the elliptical type with a convergent lens having a long focal length would then become very deep, which would make it difficult to fit in the bodywork of the vehicle.